Modify the inputstream or charstream

[mmatassoni]

Hi,
I'm doing a parser for a custom syntax and I want to provide in the
grammar an "input" command like the one of latex (\input{filename}
imports the commands from filename into the target file; it's
equivalent to typing all the commands from filename right into the
current file where the \input line is). How can I achieve this? I
thought of preprocess the inputstream/charstream in the parser constructor, but I
would like to know if I can insert the content of the file in the inputstream/char at
parse time

Thanks